Your GM does not have a timing belt like many of the imports, it has a timing chain and it does not have a regular replacement cycle. It could conceivably go several hundred thousand miles without replacement. Chances are that if it does go it will become noisy (sound a little like a diesel) which will give you a clue that a replacement is necessary.

Generally timing belt engines are freewheeling , meaning the valves will never contact the psitons if the belt fails. However, many import cars do have interference engines with timing belts and they must be repalced at 100,000 miles to be safe.